Positron-annihilation lifetime spectra have been obtained from neutron-irradiated silicon. Isochronal annealing shows that positrons trapped in divacancies have a lifetime of 325 + 20 psec and in quadrivacancies, a lifetime of 435~30 psec. From this a positron lifetime of 270 psec is predicted for monovacancies. Divacancies are found to anneal out according to a second-order process with an activation energy of 0.8~0.1 eV. Breakup of divacancies is found to follow a first-order process with an activation energy of approximately 1.7 eV. The positron trapping cross section for divacancies varied with temperature approximately as T ", which is in good agreement with theoretical calculations based on the cascade capture model of the electron capture cross section. At 296'K the positron capture cross section was estimated to be about 4X 10 ' cm'.
Position lifetime spectra and Doppler broadening spectra have been measured for 17 alkali halides. Three significant lifetime components are found in almost all cases. The results are interpreted in terms of a model in which a fraction of the positrons form positronium and the remainder are initially in a state from which either annihilation or transitions may occur. The annihilation rate from the initial state is shown to depend on the distance separating negative ions, which suggests that the positron is in a bound state.Deconvoluted Doppler broadening data indicate that at least two momentum components are present.
The spectral function for the reactmn 12C(e,e'p) HB leading to the 1/2 + state at 6.79 MeV m nB has been measured. The excitation of this non-normal panty state lndmates the presence of wave functton components beyond the lp shell A shell-model calculatmn, performed m a large configuration space, ymlds a fmr description of the shape of the momentum distribution.
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